Αυτό το παντελόνι είναι πολύ σφιχτό στη μέση.

Because παντελόνι is a neuter singular noun in Greek, and the demonstrative this has to agree with it.

• masculine: αυτός
• feminine: αυτή
• neuter: αυτό

So:

• αυτό το παντελόνι = this pair of trousers / these pants

In Greek, it is very normal to use:

demonstrative + article + noun

So:

• αυτό το παντελόνι = this pair of trousers
• literally something like this the trousers

That sounds strange in English, but it is standard Greek grammar.

Greek usually treats this item of clothing as a singular noun:

• το παντελόνι = the trousers / the pants
• τα παντελόνια = the trousers / the pants in plural, usually meaning multiple pairs

So even though English often uses a plural word, Greek commonly uses the singular for one garment.

Είναι means is. It is the 3rd person singular of είμαι = to be.

So the sentence structure is very straightforward:

• Αυτό το παντελόνι = the subject
• είναι = is
• πολύ σφιχτό = very tight
• στη μέση = at the waist / in the waist area

Unlike in some languages, Greek normally does use the verb to be in sentences like this.

Because adjectives in Greek agree with the noun they describe in gender, number, and case.

Since παντελόνι is neuter singular, the adjective also takes the neuter singular form:

• masculine: σφιχτός
• feminine: σφιχτή
• neuter: σφιχτό

So:

• το παντελόνι είναι σφιχτό
• not σφιχτός or σφιχτή

No. Here πολύ means very, so it is an adverb, and it does not change.

• πολύ σφιχτό = very tight
• πολύ καλός = very good
• πολύ ωραία = very nicely / very nice depending on context

Be careful: πολύ can also be part of the adjective/pronoun πολύς / πολλή / πολύ meaning much / many, which does change. But in this sentence, it is simply the adverb very.

Στη is the contracted form of:

σε + τη = στη

So:

• σε τη μέση becomes στη μέση

This is extremely common in modern Greek:

• στο = σε + το
• στη = σε + τη
• στον = σε + τον

Here στη μέση means at the waist or around the waist.

It can mean both, depending on context.

• η μέση can mean waist
• η μέση can also mean middle

In this sentence, because we are talking about clothing, στη μέση clearly means at the waist.

So the sentence means the trousers are tight in the waist area, not tight in the middle in some abstract sense.

Greek often uses the definite article where English might not.

So στη μέση is the natural expression here:

• στη μέση = at the waist

Saying σε μέση would sound incomplete or unnatural in this context. The article helps make it a normal, idiomatic phrase.

It is in the accusative, because after σε in Modern Greek, nouns normally appear in the accusative.

You can see that in the article:

• nominative: η μέση
• accusative: τη μέση

So:

• στη μέση = σε τη μέση

The noun μέση itself looks the same here, but the article shows the case clearly.

Yes. That would still be completely natural.

• Αυτό το παντελόνι... = This pair of trousers...
• Το παντελόνι... = The trousers / the pants...

Using αυτό makes it more specific or contrastive, as if you are pointing to this particular item.

Yes. Greek word order is more flexible than English, though not completely free.

The neutral order here is:

• Αυτό το παντελόνι είναι πολύ σφιχτό στη μέση.

But you could also hear:

• Είναι πολύ σφιχτό στη μέση αυτό το παντελόνι.

That version puts more emphasis on the description first, especially in speech. The original sentence is the most straightforward and textbook-like.

No, but it is a very natural choice here.

• σφιχτό often means tight, snug, or too tight
• στενό can also be used for clothing and often means narrow / tight-fitting

In this sentence, πολύ σφιχτό στη μέση strongly suggests the trousers feel too tight around the waist. That is a very natural way to say it.